Centrifugal fan

ABSTRACT

There is provided a centrifugal fan including a casing having an upper casing with an opening and a lower casing with a recess, a motor disposed in the recess, and an impeller housed in the casing. The impeller includes an annular shroud formed with an inlet, plural blades which are arranged between the annular shroud and the lower casing, and a hub which is connected to respective inner sides of the blades. Each of the blades includes, at a pressure surface side, a first inclined surface which is inclined toward a negative pressure surface of the blade as approaching the lower casing from the annular shroud; and a second inclined surface which is provided closer to the lower casing than the first inclined surface, and is inclined toward the negative pressure surface at an angle steeper than that of the first inclined surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2013-223462, filed on Oct. 28, 2013, the entire subject matter of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a centrifugal fan, and particularly, to a thin low-noise centrifugal fan.

2. Description of the Related Art

A centrifugal type fan (also referred to as a centrifugal fan) has been widely used for cooling, ventilation, and air conditioning of various devices such as home appliances, OA devices, and industrial devices, air blowers for vehicles, and so on.

A centrifugal fan rotates an impeller having a plurality of blades, thereby flowing air in a radial direction. In general, a centrifugal fan includes a casing Which has an inlet and an outlet, and an impeller which has a plurality of blades and is stored in the casing. The centrifugal fan rotates the impeller by rotation of a motor, thereby making air flow from the inlet through the blades and discharging the air outward in the radial direction of the impeller by a centrifugal action according to the rotation of the impeller. As a result, high-pressure air is discharged from the outlet of the casing to the outside.

JP-A-2013-47483 discloses a small-sized centrifugal fan having the above-described structure. This centrifugal fan is designed to suppress noise from being generated due to disturbance of an air flow around the inner wall surface or the outlet of a casing. Specifically, this centrifugal fan has a structure in which the casing includes an upper plate, a lower plate, and a plurality of supports interposed between the upper plate and the lower plate, and an impeller is disposed inside the casing. Side surfaces of the casing have openings to function as air outlets. A lower portion of the casing has a motor case in which a motor is mounted, and the impeller is disposed such the lower portions of the blades face the motor case with a predetermined gap. The blades are backward inclined blades and have a curved blade shape inclined while being curved backward in the rotation direction, so that the centrifugal fan configures a turbofan.

According to the centrifugal fan disclosed in JP-A-2013-47483, side walls are omitted from the casing such that openings are formed, whereby noise is suppressed from being generated due to disturbance of an air flow around the inner wall surface or the outlets of the casing. However, under a situation where devices are reduced in size and thickness, are increased in assembly density, and are reduced in power consumption, a centrifugal fan for those devices is demanded to further reduce noise.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above-described problem, and an object of the present invention is to provide a centrifugal fan capable of reducing generation of noise.

According to an illustrative embodiment of the present invention, there is provided a centrifugal fan comprising: a casing including an upper casing having an opening, a lower casing having a recess, and supports interposed between the upper casing and the lower casing; a motor disposed in the recess of the lower casing; and an impeller in the casing. The impeller includes: an annular shroud which has an upper inner portion formed with an inlet; a plurality of blades which are arranged between the annular shroud and the lower casing while protruding downward from the annular shroud; and a hub which is connected to respective inner sides of the plurality of blades. A fluid introduced from the inlet of the annular shroud is discharged to a side of the impeller according to rotation of the impeller. An upper surface of the lower casing faces the impeller and configures a portion of a wall surface which guides the fluid introduced from the inlet. Each of the plurality of blades is inclined while being curved backward in a rotation direction of the impeller as approaching an outer side of the impeller. Each of the plurality of blades includes, at a pressure surface side: a first inclined surface which is inclined to approach a negative pressure surface of the blade as approaching the lower casing from the annular shroud; and a second inclined surface which is provided closer to the lower casing than the first inclined surface, and is inclined to approach the negative pressure surface at an angle steeper than that of the first inclined surface.

In the above centrifugal fan, an angle between the second inclined surface and a plane perpendicular to a rotation axis of the impeller may be larger than 0° and smaller than 45°.

In the above centrifugal fan, an angle between the second inclined surface and a plane perpendicular to a rotation axis of the impeller may be within a range from 25° to 15°.

In the above centrifugal fan, an angle between the first inclined surface and a rotation axis of the impeller may be within a range from 4° to 8°.

In the above centrifugal fan, the first inclined surface and the second inclined surface may be connected through a curved surface having a chamfered round shape.

The above centrifugal fan may further comprise a motor which is mounted on the lower casing and is configured to rotate the impeller.

According to the above configuration, each of the blades is inclined while being curved backward in the rotation direction of the impeller as approaching the outer side of the impeller, and each of the blades has, at the pressure surface side, the first inclined surface which is inclined to approach the negative pressure surface of the blade as approaching from the annular shroud to the lower casing, and the second inclined surface which is provided closer to the lower casing than the first inclined surface, and is inclined to approach the negative pressure surface at an angle steeper than that of the first inclined surface. Therefore, a centrifugal fan capable of reducing generation of noise can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a perspective view illustrating a centrifugal fan according to an illustrative embodiment of the present invention;

FIG. 2 is a cross-sectional view taken along a line A-A of FIG. 1;

FIG. 3 is an enlarged view of a portion of FIG. 2;

FIG. 4 is a bottom view illustrating an impeller;

FIG. 5 is a sectional side view illustrating the impeller;

FIG. 6 is a view schematically illustrating a cross section of a blade; and

FIG. 7 is a graph illustrating the relation between the degree of an angle β and the noise level during rotation of the impeller.

DETAILED DESCRIPTION

Hereinafter, a centrifugal fan according to an illustrative embodiment of the present invention will be described.

FIG. 1 is a perspective view illustrating a centrifugal fan according to an illustrative embodiment of the present invention. FIG. 2 is a cross-sectional view taken along a line A-A of FIG. 1. FIG. 3 is an enlarged view of a portion of FIG. 2.

Referring to FIGS. 1 to 3, a centrifugal fan 1 includes a casing 10, an impeller 30 and a motor 60. The centrifugal fan 1 is configured such that the general shape of the centrifugal fan except for a portion where the motor 60 is mounted has a rectangular parallelepiped shape which is substantially square as seen in a plan view. The centrifugal fan 1 is a thin fan having a comparatively small dimension (height) in a vertical direction. The impeller 30 is attached to the rotor 62 of the motor 60, and is rotated by rotation of the rotor 62. According to rotation of the impeller 30, the centrifugal fan 1 discharges air introduced from an inlet 33 toward a side of the impeller 30. That is, by a hydrodynamic force due to the centrifugal action according to rotation of the impeller 30, air introduced from the inlet 33 passes through the blades 51 of the impeller 30 and is discharged outward in the radial direction of the impeller 30. The air is discharged from outlets (openings) 19 which are formed in the casing 10 and located on the sides of the impeller 30.

The motor 60 is, for example, an outer rotor type brushless motor. The motor 60 is mounted on the center portion of a motor case (an example of a lower casing) 21 by fastener members such as screws or bolts. The rotor 62 is configured by a cup-shaped rotor yoke 63 whose lower side is open, a shaft 61, and a magnet 65. The annular magnet 65 is attached to the inner surface of the rotor yoke 63. The shaft 61 is fixed to the center portion of the rotor 62.

The shaft 61 is rotatably supported by a pair of bearings 66 a mounted in a hearing holder 66. The bearing holder 66 is mounted in the motor case 21. On the outer peripheral portion of the bearing holder 66, a stator 67 is attached. The stator 67 includes stator cores which are laminated, insulators which have coils wound thereon and are mounted on the stator cores, and the like. The stator cores are disposed to face the magnet 65 in the radial direction (the left-right direction in FIG. 2) with a predetermined gap therebetween. On the stator 67, a circuit board 69 is mounted. The circuit board 69 is, for example, a printed circuit board. The circuit board 69 has various components such as electronic components for controlling the motor 60, and a drive circuit for the motor 60. The drive circuit is electrically connected to the coils.

The casing 10 is configured by assembling an upper casing 11 and the motor case 21. Specifically, the upper casing 11 and the motor case 21 are assembled with each other by fastening screws 14 which are positioned at four corners as seen in a plan view, whereby the casing 10 is configured. The upper casing 11 and the motor case 21 are assembled with each other, for example, with substantially cylindrical supports 14 b interposed therebetween at the portions where the screws 14 are arranged. A portion of each side of the casing 10 surrounded by adjacent supports 14 b, the upper casing 11, and the motor case 21 is an opening, and this opening serves as the outlet 19. In other words, the outlets 19 are formed between the upper casing 11 and the motor case 21, for example, at the sides of the casing 10, except for portions where the supports 14 b are disposed and the upper casing 11 and the motor case 21 are joined.

The screws 14 are, for example, bolts which are inserted from the motor case 21 side. Instead of the screws 14, other kinds of screws, or other kinds of joining members such as rivets may be used.

The supports 14 b may be configured integrally with any one of the upper casing 11 and the motor case 21. For example, the supports 14 b may be formed integrally with the upper casing 11, and the supports 14 b and the motor case 21 may be joined by the screws 14 of the like. Alternatively, the supports 14 b may be formed together with the motor case 21 by sheet-metal processing, and the supports 14 b and the upper casing 11 may be joined by the screws 14 or the like.

The impeller 30 is disposed to be stored in the casing 10. The upper casing 11 is disposed on the impeller 30, and the motor case 21 is disposed below the impeller 30. That is, the centrifugal fan 1 is configured by holding the impeller 30 by the upper casing 11 and the motor case 21, such that the impeller 30 is disposed between the upper casing 11 and the motor case 21.

The impeller 30 generally includes an upper annular shroud 31, a lower hub 41, and a plurality of blades 51. At the center portion of the impeller 30, the inlet 33 is formed to be open toward the upper side. The inlet 33 is formed to be surrounded by an upper end portion 35 on the inner side of the annular shroud 31. The plurality of blades 51 are arranged at appropriate intervals in a circumference direction. The blades 51 are disposed between the annular shroud 31 and the hub 41, that is, between the annular shroud 31 and the motor case 21 while protruding downward from the annular shroud 31. An inner portion of each blade 51 is connected to the hub 41.

Each blade 51 has the same curved shape. That is, each blade 51 has a shape inclined backward in the rotation direction. Specifically, each blade 51 is inclined while being curved backward in the rotation direction of the impeller 30 as approaching the outer side of the impeller 30. In FIGS. 1 to 3, the shapes of the blades 51 are simply shown. The specific shapes of the blades 51 will be described below. The annular shroud 31, the hub 41, and the blades 51 may be integrally formed of a resin such as engineering plastic by molding.

The hub 41 has a cylindrical portion 43 at a center portion thereof. The rotor 62 is inserted into the cylindrical portion 43, whereby the impeller 30 is held.

The upper casing 11 is formed of, for example, a resin such as engineering plastic. The upper casing 11 has an opening 13 at a center portion thereof. The opening 13 is circular as seen in a plan view. The opening 13 is formed such that air is introduced into the inlet 33 of the impeller 30. The opening 13 has an inside diameter slightly larger than that of the inlet 33 formed by the annular shroud 31. At the circumferential edge of the opening 13, a protruding part is formed to protrude downward, and the inner surface of the protruding part and the outer periphery surface of the annular shroud 31 positioned in the vicinity of the upper end portion 35 are disposed with a slight gap therebetween.

In an upper surface of the upper casing 11, a plurality of recesses 11 b are formed as thickness reduction parts, and between adjacent recesses 11 b, ribs 11 a are formed. Since the ribs 11 a and the recesses 11 b are formed as described above, the upper casing 11 is comparatively light, and is sufficiently rigid.

The motor case 21 is formed of, for example, a plate of a metal such as iron. Similarly to the casing 10, the motor case 21 has a rectangular shape. At the center portion of the motor case 21, a recess 23 is formed downward. The recess 23 is formed in a bowl shape. As shown in FIG. 2, in the present illustrative embodiment, the motor 60, and the drive circuit for the motor 60 such as the circuit board 69 are installed in the recess 23. The motor 60 is mounted on the motor case 21 by fastening members such as screws and bolts; however, the motor 60 may be mounted on the motor case 21 while the lower portion of the bearing holder 66 is fixed to the recess 23 by caulking, instead of the fastening members.

An outer circumferential portion of the motor case 21 forms a side plate bent in an axial direction (in an upper-lower direction of FIG. 2). By providing the side plate, the rigidity of the motor case 21 can be improved.

At an upper surface of the motor case 21, a portion around the recess 23 forms. a partition portion 29 facing a lower surface of the impeller 30. The partition portion 29 is formed in a planar shape to be close to the lower surface of the impeller 30.

As shown in FIG. 2, the huh 41 of the impeller 30 is provided only at a portion close to the shaft (the rotation axis of the impeller 30) 61 such that at least an outer circumferential side portion of each blade 51 faces the partition portion 29. That is, the blades 51 face a portion of the impeller 30 facing the partition portion 29. The partition portion 29 configures a portion of a wall surface which guides air introduced from the inlet 33 to the sides. The blades 51 are disposed to face the partition portion 29 with a predetermined gap in the axial direction. Incidentally, the lower portion of each blade 51 may partially or entirely face the partition portion 29.

As shown in FIG. 3, a portion of the upper surface of the hub 41 configures a curved surface 49 which forms a curved line having an arc shape convex downward in a lateral cross section. An outer circumferential end portion 45 of the hub 41 is positioned in the vicinity of the lower side from the upper end portion 35 of the upper shroud 31 in the vertical direction. Also, an inner circumferential end portion 47 of the hub 41 is positioned in the vicinity of an upper end portion 63 a of the outer periphery of the rotor yoke 63. The curved surface 49 is formed between the outer circumferential end portion 45 and the inner circumferential end portion 47. The lowermost portion of the curved surface 49 is the outer circumferential end portion 45.

The dimension of the outside diameter of the impeller 30 which is stored in the casing 10 is set to be smaller than the dimension of one side of the casing 10. Therefore, the impeller 30 does not protrude from the outer peripheral edge of the casing 10 when rotating, and thus contact of the impeller 30 with other members, damages due to contact, and the like are prevented.

The motor case 21 also serves not only as a main plate for guiding air in the impeller 30, but also as a base plate of the casing 10. For this reason, setting of the gap to be formed between the impeller 30 and the partition portion 29 may be important. In a case where the gap is excessively large, air suctioned from the inlet 33 flows into the gap while passing through the blades 51. As a result, the pressure of air discharged from the impeller is reduced, and thus blowing characteristics are reduced. Meanwhile, in a case where the gap is excessively small, there is the following problem. That is, if a variation occurs in the accuracy of the dimensions of each component, there is a possibility that the blades 51 will come into contact with the partition portion 29. In order to prevent this contact, it is necessary to manage the accuracy of the dimensions of each component with a high precision, and thus the manufacturing cost of the centrifugal fan 1 would increase. In view of those problems, the gap between the impeller 30 and the partition portion 29 is appropriately set.

Subsequently, the structure of the impeller 30 will be described in more detail.

FIG. 4 is a bottom view illustrating the impeller 30. FIG. 5 is a sectional side view illustrating the impeller 30.

Referring to FIGS. 4 and 5, the impeller 30 is a thin impeller which has a substantially disk shape. Therefore, the centrifugal fan 1 can be configured to be thin. As shown in FIG. 4, in the impeller 30, for example, seven blades 51 are arranged. The inside diameter D1 of the upper end portion 35 of the annular shroud 31 is larger than the outside diameter D2 of the hub 41. Therefore, separate upper and lower molds can be used to manufacture the impeller 30. Therefore, the plurality of blade 51, the annular shroud 31, and the hub 41 can be integrally formed of a resin such as engineering plastic, as one component, for example, by injection molding.

Each blade 51 has a pressure surface 53 and a negative pressure surface 54. The pressure surface 53 faces the front side of the impeller 30 in the rotation direction (a counterclockwise direction, that is, a direction shown by an arrow R in FIG. 4). The negative pressure surface 54 faces the opposite side to the pressure surface 53.

The blades 51 are backward inclined blades, and are of a so-called turbo type. The blades 51 have a blade shape inclined while being curved backward in the rotation direction. The specific shape of each blade 51 is, for example, as follows. That is, as the pressure surface 53 is seen from a direction in which the rotation axis of the impeller 30 extends (as the pressure surface 53 is seen in a bottom view), the pressure surface 53 has roughly a shape in which three arcs are connected. These arcs are connected such that adjacent arcs are tangent to each other. Therefore, the flow and static pressure of the centrifugal fan 1 can be increased and noise of the centrifugal fan 1 can be reduced. The negative pressure surface 54 has a curved shape roughly following the pressure surface 53 such that the distance from the pressure surface 53 is reduced as the distance from the rotation axis of the impeller 30 increases, as seen in a bottom view. However, the shapes of the blades 51 are not limited thereto.

An edge of each blade 51 on the rotation axis side of the impeller 30, that is, the inlet 33 side becomes a leading edge, and an edge of each blade 51 on the peripheral side surface side of the impeller 30 becomes a trailing edge. As shown in FIG. 5, the leading edge of each blade 51 is configured in a tapered shape such that the leading edge approaches the rotation axis of the impeller 30 as approaching the hub 41 from the annular shroud 31. The trailing edge of each blade 51 has a shape substantially perpendicular to the rotation axis of the impeller 30.

FIG. 6 is a view schematically illustrating a cross section of a blade 51.

The cross section shown in FIG. 6 is a cross section which is perpendicular to a horizontal plane perpendicular to the rotation axis and is substantially perpendicular to the pressure surface 53 as seen in a bottom view. That is, the cross section shown in FIG. 6 corresponds to a cross section along the line C-C of FIG. 4. In FIG. 6, hatching is omitted. An arrow Z indicates a direction (upper side) parallel to the rotation axis of the impeller 30.

In the present illustrative embodiment, as shown in FIG. 6, the thickness of each blade 51, that is, the distance between the pressure surface 53 and negative pressure surface 54 of each blade 51 decreases as the distance from the annular shroud 31 in a direction parallel to the rotation axis increases. In other words, each blade 51 is formed to become thinner as approaching the partition portion 29. Therefore, the distance between the pressure surface 53 of the blade 51 and the negative pressure surface 54 of an adjacent blade 51 increases as approaching the partition portion 29.

The negative pressure surface 54 is substantially parallel to the rotation axis of the impeller 30. Strictly, the negative pressure surface 54 has a draft angle, and thus is not perfectly parallel to the rotation axis. However, since this draft angle is a small angle, the inclination angle of the negative pressure surface 54 is smaller than the inclination angle of the pressure surface 53.

In contrast to this, the pressure surface 53 includes a first inclined surface 53 a, a second inclined surface 53 b, and a chamfered surface 53 c. Also, the entire of the pressure surface 53 of each blade 51 may be configured by the three surfaces 53 a, 53 b, and 53 c, or may further include any other surfaces.

The first inclined surface 53 a is inclined toward the negative pressure surface 54 of the blade 51 as approaching the motor case 21 from the annular shroud 31, and an upper portion of the first inclined surface 53 a is connected to the annular shroud 31.

The second inclined surface 53 b is provided closer to the motor case 21 than the first inclined surface 53 a. In the present illustrative embodiment, the second inclined surface 53 b is connected to the first inclined surface 53 a through the chamfered surface 53 c, and extends to the lower end portion of the pressure surface 53, that is, the lower end portion of the blade 51. The second inclined surface 53 b is provided throughout every blade 51 from the inner side to the outer side (from the hub 41 side to the front end portion).

Here, the second inclined surface 53 b is inclined at an angle steeper than that of the first inclined surface 53 a so as to approach the negative pressure surface 54. That is, if the angle between the first inclined surface 53 a and the rotation axis of the impeller 30 is referred to as α, and the angle between the second inclined surface 53 b and a plane (that is, a horizontal plane) perpendicular to the rotation axis of the impeller is referred to as β, as shown in the cross section of FIG. 6, the angle β is smaller than an angle Obtained by subtracting the angle α from 90°.

The chamfered surface 53 c is formed between the first inclined surface 53 a and the second inclined surface 53 b so as to round a ridge line formed by the two surfaces 53 a and 53 b. In the present illustrative embodiment, the chamfered surface 53 c is a curved surface having a chamfered round shape (for example, a curved surface whose cross section has substantially an elliptical arc shape or a circular arc shape), and is, for example, a curved surface having a predetermined curvature.

The angle α is set within a range from 4° to 8°. With this range, noise can be efficiently reduced, without reducing the air flow characteristics so much.

The angle β is set to be larger than 0° and smaller than 45°. Preferably, the angle β is set within a range from 25″ to 35°.

FIG. 7 is a graph illustrating the relation between the degree of the angle β and the noise level during rotation of the impeller 30.

In FIG. 7, the horizontal axis and the vertical axis correspond to the degree of the angle β and the noise level, respectively FIG. 7 shows rotational noise (nz noise) values measured as noise levels in cases where the angle α is a predetermined angle in the range from 4° to 8°) and the angle β is 15°, 25°, 35°, 45°, or 55°. Rotational noise (nz noise) is noise depending on the product of the rotation speed n (rpm) of the fan (the rotation speed of the impeller 30) and the number z of blades, and is generated according to the rotation speed of the impeller 30. Therefore, sound pressures at the frequency (nz/60) (Hz) of rotational noise were measured and compared. During measurement in each case, the rotation speed of the impeller 30 was set such that a predetermined static pressure was obtained.

It can be seen that as shown in FIG. 7, as compared to a case where the angle β is 0° (a case where the angle β is not formed and only the angle α is formed, that is, a case where the second inclined surface 53 b does not exist and only the first inclined surface 53 a exists), if the angle β is set to be smaller than 45°, the rotational noise (nz noise) can be efficiently reduced. Further, following a reduction of rotational noise (nz noise), the rotational noise (knz noise; k=2, 3, . . . ) which are the harmonics of the rotational noise can be reduced. Especially, it can he seen that if the angle β is set within a range from 25° to 35°, the rotational noise (nz noise) can be more efficiently reduced.

If the angle β is set to be larger than 45°, the reduction effect of the rotational noise (nz noise) decreases.

As described above, in the present illustrative embodiment, since not only the first inclined surface 53 a but also the second inclined surface 53 b having the angle β smaller than 45° are provided in the pressure surface 53 of each blade 51, a high static pressure can be secured while reducing the rotational noise (nz noise) and the harmonics of the rotational noise. It is considered that this effect can be achieved because generation of separated flows from the pressure surfaces 53 in the vicinities of the outlets 19 where the wind velocity increases can be reduced.

[Others]

The pressure surface may not be formed with a chamfered surface. For example, the pressure surface side of each blade may have a first inclined surface which is gradually inclined toward the negative pressure surface side of the blade as approaching the motor case from the annular shroud, and a second inclined surface which is inclined from a middle position of the first inclined surface toward the negative pressure surface at an angle larger than that of the first inclined surface.

Between the first inclined surface and the second inclined surface, one or more inclined surfaces each having an angle different from the inclination angle of the first inclined surface and the inclination angle of the second inclined surface may he provided.

The pressure surface may have the above-described inclined surfaces only in a part of the area of the blade from the inner side to the outer side. Also, at the lower end portion (partition portion side) of each blade, the pressure surface may he substantially perpendicular to a horizontal plane similarly to the negative pressure surface, and only a portion of the pressure surface close to the upper shroud may be tapered. Also, the pressure surfaces of only some blades of the plurality of blades may be tapered.

The pressure surfaces of the blades are not limited to tapered pressure surfaces as linearly shown in a cross section as shown in FIG. 6. For example, the first inclined surface or the second inclined surface may be formed so as to slightly curve while approaching the negative pressure surface as approaching the partition portion, in a cross section as described above.

The rough shape of the pressure surface of each blade as seen in a bottom view may not be a shape in which three arcs are connected as described above, and may not be a shape expressed by a combination of high-dimensional functions passing three points. The blades need only to be formed in an appropriate shape satisfying a desired condition.

The shape of the casing is not limited to a substantially square shape as seen in a plan view. The casing may have any arbitrary shapes such as a polygonal shape, a circular shape, and an asymmetric shape. The portions where the upper casing and the motor case are joined with each other are not limited to the insides of four corners of the upper casing as seen in a plan view. For example, at portions connected to the upper casing so as to protrude outward from the outer peripheral edge forming a substantially square shape as seen in a plan view of the upper casing, screws, supports, and the like for joining the upper casing and the motor case may be provided.

Also, in a case where the supports are provided between the upper casing and the motor case at the portions where the upper casing and the motor case are joined with each other, the shapes of the supports may be, for example, as follow. That is, the supports may have a substantially cylindrical shape having a size allowing screws for joining the upper casing and the motor case to pass through. If supports having a shape as described above are used, air discharged from the impeller is discharged from the sides of the casing to the outside, with little or no resistance. Therefore, noise of the centrifugal fan can be reduced.

The motor case may be formed by using materials such as a resin material other than a metal plate. The upper casing and the motor case may be formed integrally. Instead of the motor case, a lower casing, having no space for mounting the motor may be used such that the upper casing and the lower casing configures the casing. That is, the centrifugal fan is not limited to the configuration in which the motor is mounted on the lower casing.

It should be noted that the above-mentioned illustrative embodiment is merely illustrative in all aspects and are not to be construed as limiting the invention. The scope of the invention is defined by the appended claims rather than the detailed description of the invention. All changes or modifications or their equivalents made within the meanings and scope of the claims should be construed as falling within the scope of the invention. 

What is claimed is:
 1. A centrifugal fan comprising: a casing including an upper casing having an opening, a lower casing having a recess, and supports interposed between the upper casing and the lower casing; a motor disposed in the recess of the lower casing; and an impeller housed in the casing, wherein the impeller includes: an annular shroud which has an upper inner portion formed with an inlet; a plurality of blades which are arranged between the annular shroud and the lower casing while protruding downward from the annular shroud; and a hub which is connected to respective inner sides of the plurality of blades, wherein a fluid introduced from the inlet of the annular shroud is discharged to a side of the impeller according to rotation of the impeller, wherein an upper surface of the lower casing faces the impeller and configures a portion of a wall surface which guides the fluid introduced from the inlet, wherein each of the plurality of blades is inclined while being curved backward in a rotation direction of the impeller as approaching an outer side of the impeller, and wherein each of the plurality of blades includes, at a pressure surface side: a first inclined surface which is inclined toward a negative pressure surface of the blade as approaching the lower casing from the annular shroud; and a second inclined surface which is provided closer to the lower casing than the first inclined surface, and is inclined toward the negative pressure surface at an angle steeper than that of the first inclined surface.
 2. The centrifugal fan according to claim 1, wherein an angle between the second inclined surface and a plane perpendicular to a rotation axis of the impeller is larger than 0° and smaller than 45°.
 3. The centrifugal fan according to claim 1, wherein an angle between the second inclined surface and a plane perpendicular to a rotation axis of the impeller is within a range from 25° to 35°.
 4. The centrifugal fan according to claim 1, wherein an angle between the first inclined surface and a rotation axis of the impeller is within a range from 4° to 8°.
 5. The centrifugal fan according to claim 1, wherein the first inclined surface and the second inclined surface are connected through a curved surface having a chamfered round shape.
 6. The centrifugal fan according to claim 1, wherein the motor is configured to rotate the impeller. 